Field Of The Invention
The invention relates to a method of drilling a part, in particular a turbine engine part, by means of a pulse laser.
Description Of The Related Art
Numerous turbine engine parts are drilled in order to form orifices, e.g. for passing a flow of cooling air. This applies in particular to turbine vanes and blades, and also to combustion chamber walls, which walls may include several thousands of cooling orifices.
In order to ensure effective cooling of these parts and in order to avoid shortening their lifetimes, the drilled orifices must present specific shapes, and they must not present microcracking on their internal walls. Furthermore, the technology used for drilling must affect the material soundness of the parts as little as possible.
Three different technologies are known for drilling a turbine engine part for the purpose of forming cooling orifices: drilling by means of a sharp tool, electrical discharge machining (EDM), and laser pulse drilling.
Drilling by means of a sharp tool is not appropriate for making orifices of small diameter (e.g. lying in the range 0.4 millimeters (mm) to 1.5 mm). EDM drilling is suitable for making orifices of small size. Nevertheless, because that type of drilling is relatively expensive to implement, it is used only for drilling parts having a small number of cooling orifices.
Drilling by means of laser pulses (e.g. using a YAG type laser) also makes it possible to drill orifices of small size. Nevertheless, prior art laser devices are used for making only a small number of successive orifices, since they are not stable, so the quality and the repeatability of the shapes of the orifices decrease after such devices have been in use for a certain length of time. By way of example, it has been found that when a laser drilling device is used for making orifices of circular section, the orifices that are drilled after the device has been in use for a certain length of time present a section that is somewhat elliptical. That device is therefore used for relatively short durations only, and it needs to be rested for relatively long periods of time between two successive uses.
A pulse laser generally comprises a cavity having mounted therein a solid bar for generating laser pulses. Typically, the bar is elongate in shape and forms an amplifying medium into which photons are emitted from at least one flash lamp housed in the cavity. Two mirrors, one of which is partially reflecting, are arranged at the longitudinal ends of the bar. The laser cavity also has a temperature sensor that is connected to detection means for warning an operator when the temperature of the cavity reaches a certain threshold. In order to avoid the laser cavity reaching the threshold, it is cooled while it is in operation.
In the prior art, the temperature of the laser cavity is thus not taken into account for controlling or programming the laser generator. The above-mentioned cavity threshold temperature is constant, regardless of the characteristics of the part or of the orifices to be drilled, and it is determined merely for the purpose of guaranteeing an optimum lifetime for the laser generator.